The present invention relates generally to field emission devices, and more particularly, to field emission displays having current-limiting resistors.
A typical field emission display 8 is shown in FIG. 1. The display 8 includes a substrate or base plate 10 having a conductive layer 12 formed thereon. A plurality of emitters 14 are formed on the layer 12. Also formed on the layer 12 is an electrically insulating layer 16 having a conductive layer formed thereon. The conductive layer formed on the insulating layer 16 typically functions as an extraction grid 18 to control the emission of electrons from the emitters 14, and is typically formed from metal. An anode 20, which acts as a display screen and has a cathodoluminescent coating 22 formed on an inner surface thereof, is positioned a predetermined distance from the emitters 14. Typically, a vacuum exists between the emitters 14 and the anode 20. A power source 24 generates a voltage differential between the anode 20 and the substrate 10, which acts as a cathode. Also, a voltage applied to the extraction grid 18 generates an electric field between the grid and the substrate 10. An electrical path is provided to the emitters 14 via the conductive layer 12 such that in response to this electric field, the emitters 14 emit electrons. The emitted electrons strike the cathodoluminescent coating 22, Which emit light to form a video image on the display screen. Examples of such field emission displays are disclosed in the following U.S. patents, all of which are incorporated by reference:
Field emission displays, such as the field emission display 8 of FIG. 1, often suffer from technical difficulties relating to the control of the current flowing through the emitters 14. For example, due to the relatively small dimensions of the components involved, manufacturing defects are common in which an emitter 14 is shorted to the extraction grid 18. Because the voltage difference between the substrate 10 and the anode 20 is typically on the order of 1000 volts or more and a high electric field exists between tip 14 and substrate 10, the above defect can cause a current to flow through the emitter 14 that is sufficient to destroy not only the shorted emitter 14 itself, but other surrounding emitters 14 and circuitry as well. Thus, such a current draw will typically result in damage to, if not complete destruction of, the field emission display. Furthermore, if the current through the emitters 14 is unregulated, it is virtually impossible to control the emission level of the emitters 14, and thus the brightness level of the field emission display 8.
Efforts to solve the above limitations have focused on providing a resistance between the conductive layer 12 and the emitters 14 to limit the current flow through the emitters 14. An example of such a resistance is disclosed in U.S. Pat. No. 4,940,916, which was previously incorporated by reference. One limitation to this scheme, however, is that the resistivity (which is the inverse of the conductivity) of the resistive layer often fluctuates in response to conditions that vary during the operation of the field emission display, particularly the varying light intensity resulting from the emitted electrons striking the cathodoluminescent coating 22 or from ambient light.
According to one aspect of the present invention, a semiconductor structure is provided for use in a field emission display. The structure includes a substrate that may be formed from a semiconductor material, Corning glass, soda lime glass, plastic, or silicon dioxide. A first layer of a conductive material is formed on the substrate. A second layer of microcrystalline silicon is formed on the conductive layer. One or more cold-cathode emitters are formed on the second layer. The second layer forms a current-limiting resistance between the conductive layer and the emitters.
In one aspect of the invention the second layer, while exposed to optical energy, exhibits a resistivity that differs less than approximately 10% from the resistivity of the second layer while it is unexposed to optical energy, or xe2x80x9cin the dark.xe2x80x9d
In further aspects of the invention, the second layer of microcrystalline silicon is doped with an impurity of either the p-type or the n-type.
An advantage provided by one aspect of the present invention is a current-limiting resistor that has a resistivity that remains relatively stable while the resistor is exposed to varying light intensities.